1. Field of the Invention
The present invention relates to a power fastening tool such as an impact driver or an impact wrench used for fastening a fastening member such as a screw or a bolt.
2. Description of the Related Art
Conventionally, a power fastening tool used for fastening a fastening member such as a screw or a bolt utilizing driving force of a motor has a function automatically stopping driving of the motor when a torque necessary for fastening the fastening member reaches to a predetermined value due to the fastening member is fastened completely.
In a first conventional power fastening tool such as an impact wrench used for fastening a fastening member, for example, shown in publication gazette of Japanese Patent Application 4-322974, a number of impact of a hammer is sensed and driving of a motor is automatically stopped when the number of impact reaches to a predetermined reference number.
In a second conventional power fastening tool shown in publication gazette of Japanese Patent Application 9-285974, a rotation angle of a fastening member is sensed, and driving of a motor is stopped, when the rotation angle reaches to a predetermined reference angle.
In a third conventional power fastening tool shown in publication gazette of Japanese Patent Application 6-91551, an actual torque, which is necessary for fastening a fastening member, is sensed and driving of a motor is stopped when the actual torque reaches to a predetermined reference value.
The first conventional power fastening tool which stops the driving of the motor corresponding to the impact number and the second conventional power fastening tool which stops the driving of the motor corresponding to the rotation angle respectively have a disadvantage that a large difference may occur between a desired torque and the actual torque for fastening the fastening member. The difference causes loosening of the fastening member due to insufficient torque when the actual torque is much smaller than the desired torque. Alternatively, the difference causes to damage the elements to be fastened by the screw or the bolt or to damage a head of the fastening member due to superfluous torque when the actual torque is much larger than the desired torque.
On the other hand, the third conventional power fastening tool which stops the driving of the motor corresponding to the actual torque for fastening the fastening member needs a sensor provided on an output shaft for sensing the actual torque, so that it causes the cost increase and the upsizing of the power impact tool, even though the automatic stopping of the driving of the motor can be controlled precisely corresponding to the actual torque.
For solving the above-mentioned problems, in a fourth conventional power fastening tool shown in publication gazette of Japanese Patent Application 2001-277146, a torque for fastening the fastening member is estimated according to rotation speed of a shaft of a motor or impact energy of a hammer. It is judged that the fastening member is fastened completely when the estimated torque is suddenly increased. The driving of the motor is stopped when the estimated torque is suddenly increased.
In the fourth conventional power fastening tool, there is a possibility that the driving of the motor is stopped before the fastening member is fastened completely, when the torque is temporarily increased due to trouble in fastening operation. In order to prevent the stop of the driving of the motor before the fastening member is fastened completely, it is considered that the motor is stopped only when the value of the torque becomes larger than a predetermined threshold value. In the actual fastening operation, since the bolts, the nuts, and so on having various diversiform or made of various materials can be used as the fastening member, it is difficult to stop the rotation of the motor surely at the moment when the fastening member is fastened completely by setting one threshold value with respect to the torque for fastening the fastening member.
As examples of cause of the trouble in the fastening operation, an irregularity of thread grooves of the bolt, a slight warp of a member interleaved between the bolt and the nut, decentering of the bolt or the nut, clipping of dust between the bolt and the nut, picking off of a surface coating of baking finish on the bolt, lifting of a member inter leaved between the bolt and the nut, are recited.